Camping holidays can be the cheapest way to get away as a family. You can often pay less than £10 per night for a very basic site and around £20-£25 for a site that has more facilities. IF you have your own tent, you can often book a campsite quite last minute, although during summer holidays, popular sites and locations do tend to book up fast so if you want a pitch with electric hook up or an extra large pitch for example, you may need to book a few weeks in advance.
If you are not sure exactly where you want to go you can use search sites such as Pitch Up which allows you to easily compare a number of sites based on location, price or facilities. You can enter the dates you want to go and then select as many or as few filters as you like to narrow the results down.
You can often book the site directly through the website often paying a small deposit with the balance due a week or so before you go.
Always read plenty of reviews for a site before booking so you can get a feel for what it is like and what the facilities are like.